Lactase is a brush border enzyme that hydrolyses lactose.  Its concentration declines steadily as one ages into adulthood, especially in people of non-European ancestry.  When there is an inability to absorb lactose found in milk and dairy products, the condition is called lactose intolerance.  Acquired deficiency is most commonly seen in Asian (90%) and African populations.  Patients typically manifest with osmotic diarrhea, abdominal cramps, bloating and flatulence after ingestion of such products.

Previously, the lactose tolerance test was used to aid in the diagnosis of patients.  This test is based on measurement of the blood glucose level after oral lactose administration.  The lactose tolerance test is cumbersome and time consuming.  The lactose hydrogen breath test, a noninvasive test, can confirm the diagnosis if unclear.  A positive hydrogen breath test is characterized by a rise in the measured breath hydrogen level after the ingestion of lactose, thus indicating bacterial carbohydrate metabolism (ie, impaired malabsorption).

(Choice A)  Urine test for reducing substances is positive in patients with glucosuria, galactosuria, and so on.  Those are not characteristic features of lactose intolerance, which is characterized by a positive stool (rather than urine) test for reducing substances due to carbohydrate malabsorption.

(Choice B)  The diarrhea secondary to lactase deficiency has a high osmotic gap (eg, >125 mOsm/kg), due to the unmetabolized lactose and organic acids.  The osmotic gap is calculated as 290 - [2 (stool Na + stool K)] and is >75 mOsm/kg in all forms of osmotic diarrhea.

(Choice D)  The stool pH is acidic in lactase deficiency due to the fermentation products.

(Choice E)  Acid steatocrit is a test for fat malabsorption, not carbohydrate malabsorption.

Educational objective:
Lactose intolerance is characterized by a positive hydrogen breath test, positive stool test for reducing substances, low stool pH and increased stool osmotic gap.  There is no steatorrhea.